This invention relates to photosensor array apparatus, and more particularly to photosensor array apparatus in which the photoresponse speed of a photosensor is improved.
In photosensor array apparatus used at a light input portion of a facsimile, letter reader or the like, a so-called line sensor is generally used in which a plurality of photosensors are arranged along a straight line for reading picture images. By driving, at a high speed, such a line sensor, light reflected from a picture is converted into electric signals for improving read out efficiency.
Recently, as a photosensor, an i-type amorphous silicon diode made of an i-type amorphous silicon film having a photoconductive effect has been proposed because it strongly absorbs light reflected from the picture and can use low cost substrate material. Where such an i-type amorphous silicon diode is used as the photosensor, its response speed is about 0.1-1 ms when the intensity of light from a light source is about 100 luxes. When the amorphous silicon diode is applied to a facsimile, the time required for reading one line is about 5-10 ms so that about 20 seconds are required to read a manuscript having a size of A4 (210 mm.times.297 mm). Although this read time is a standard value for a present time high speed facsimile device, in recent years it is desirable to increase the photoresponse speed of the photosensor with increase in the operating speed of the facsimile device. Where the line sensor made of the i-type amorphous silicon is driven at a high speed, the photoresponse speed presents a problem. More particularly, when light having energy larger than the energy gap between conduction band and valence band of amorphous silicon is irradiated, the electrons in the valence band migrate to conduction band, leaving holes. When either electrons or holes migrate as majority carriers, minority carriers are trapped temporarily to delay the photoresponse speed. For example, in photosensor array apparatus where electrons act as the majority carriers, holes are trapped so that it takes several milliseconds for recovery to an original dark current level with error of less than about 1% when light is off, thus degrading the residual image characteristic and delaying the light response characteristic.